1. Field of the Invention
The present invention relates to a microlens that is formed by processing a substrate such as a silicon substrate, a method of manufacturing same, and a photomask suitable for use in the manufacturing method.
2. Description of Related Art
The transition to broadband communications in recent years invites the remarkable increase in the amount of traffic. Correspondingly, the popularization of optical communications in which light is used as the medium has been rapid.
Further, where optical modules for sending and receiving optical signals are concerned, the demand for a bi-directional optical communication module such as an optical transceiver, a micro BOSA (Bi-directional Optical Sub-Assembly) or an infrared sensor has increased.
The additional demand with respect to miniaturization and lower costs for such optical modules increases more and more.
With the object of further miniaturizing the optical module, cases where a silicon microlens is adopted have been on the increase. The silicon microlens can be formed by using silicon micro fabrication in particular, which is adopted in the manufacturing process of semiconductor devices.
Japanese Patent No. 3117886 to OKI Electric Industry Co., Ltd. discloses a method of manufacturing such a silicon microlens. The disclosed method employs a mask used for resist pattern for manufacturing a lens. The mask comprises a plurality of mask patterns in which light-shielding regions that are provided in a concentric circle shape or concentric polygonal shape and that separated by gaps serving as light transmission regions on a mask substrate. The width of the gap between two adjacent mask patterns increases at least stepwise moving from the center point of the concentric circle or concentric polygon toward the outer perimeter in the radiation direction. Further, the pitch in the radial direction of the mask pattern is smaller than the length corresponding to a resolution limit of an optical system of an aligner in which the mask is being incorporated. By using the mask, a resist pattern is obtained by exposing a resist on a lens formation substrate by controlling the intensity of light projected from an aligner. The lens is manufactured with the shape of the resist pattern transferred by means of etching process using the resist pattern.
Furthermore, Japanese Patent Application Laid Open No. 2004-85873 (corresponding to U.S. Pat. No. 7,027,693) discloses an optical module including a support substrate and an optical member having formed there at the aforementioned silicon microlens. The optical member is mounted on the support substrate. In that case, the optical member is in contact with the support substrate at a portion of the surface of the optical member facing the support substrate and in non-contact with the support substrate at a remaining portion of the surface with an adhesive material being filled between the non-contact surface portion and the support substrate.
However, according to the above described method of manufacturing a microlens, the minimum pitch, the minimum dimension width and minimum dimension width increment of the mask patterns of a used photomask are subjected to limitations depending on the resolving and dimensional control properties of the aligner. Here, the width increment means the width difference between a width w1 of one mask pattern and a width w2 of the other mask pattern. Accordingly, an applicable lower limit value exists for the minimum pitch, the minimum dimension width and the minimum dimension width increment.
Furthermore, in the aforementioned method, after forming of microlens by processing a substrate, the substrate is subjected to a photolithography step and an etching step at regions outside the microlens region. In that case, the thickness of the resist pattern applied directly onto the microlens is thinner than the thickness in the regions outside the microlens region.
Hence, the upper surface of the microlens is scraped off as a result of the etching process and there is a risk that the optical characteristics of the microlens thus formed will be damaged.
By the way, because the microlens has a minute structure, so that, for facilitating the mounting of a microlens on a support substrate, it is desirable to form a microlens structure having a lens body and a frame for supporting it.
Therefore, the present inventor discovered a technique for forming a photomask that is used to form, through etching, a convex lens as a lens body, using this photomask to form a convex lens in only the first etching processing, and using another photomask to perform a second etching process to form the frame around the convex lens.